EP3482175A1 - Infrared upconversion spectrometer for the mid-ir range - Google Patents
Infrared upconversion spectrometer for the mid-ir rangeInfo
- Publication number
- EP3482175A1 EP3482175A1 EP17739937.5A EP17739937A EP3482175A1 EP 3482175 A1 EP3482175 A1 EP 3482175A1 EP 17739937 A EP17739937 A EP 17739937A EP 3482175 A1 EP3482175 A1 EP 3482175A1
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- Prior art keywords
- light
- nonlinear material
- upconverted
- infrared
- laser
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
- G01J3/108—Arrangements of light sources specially adapted for spectrometry or colorimetry for measurement in the infrared range
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
- G01J3/0245—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using an optical amplifier of light, e.g. doped fiber
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
- G01J3/0208—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/12—Generating the spectrum; Monochromators
- G01J3/18—Generating the spectrum; Monochromators using diffraction elements, e.g. grating
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/2803—Investigating the spectrum using photoelectric array detector
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/2823—Imaging spectrometer
Definitions
- the present invention relates to spectral analysis of infrared electromagnetic radiation. More specifically, the invention relates to an infrared upconversion spectrometer and a dispersive element. BACKGROUND OF THE INVENTION
- FTIR Fourier Transform Infrared
- Common FTIR spectrometers must scan a reference mirror with very high precision on a centimetre scale, requiring an extremely high precision mechanical system, with associated high costs, non-instantaneous measurements and generally a low tolerance for vibrations.
- detection of the radiation is commonly performed with mid-IR detectors. It has been shown in "Room-temperature mid-infrared single-photon spectral imaging" (Jeppe Seidelin Dam, et al., Nature Photonics 6, pp.
- a mid-IR spectral measurement based on frequency up-conversion was described in "High-resolution mid-IR spectrometer based on frequency upconversion", Qi Hu, et al., Optics Letters, 37(24), pp. 5232-5234, 2012. Upconversion spectral measurements are based on shifting the spectrum from the mid-IR region to the near visible spectral region where detectors are better developed and there are less thermal noise. The frequency shift is obtained by sum frequency mixing with a laser, resulting in a simple shift of the frequency while maintaining the spectral content for subsequent detection. In this reference, a wavelength range from about 2.89 ⁇ -3.00 ⁇ ⁇ was up-converted using three different temperatures of the nonlinear crystal to phase-match different wavelength ranges.
- a drawback of temperature tuning the nonlinear crystal is that slow temperature changes must be used to avoid damage to coatings on end faces of the nonlinear crystal .
- acquisition time of a full spectrum in the described spectral measurement will be on the order of minutes.
- the temperature tuning will typically also change the laser cavity performance in an intracavity setup and degrade the mixing laser field due to thermal expansion of the nonlinear crystal.
- a relatively narrow wavelength range was detected .
- WO 2015/003721 discloses a multichannel infrared upconversion spectrometer with several channels, each channel configured to upconvert different wavelength ranges. By using two or more up-conversion channels, an extended input wavelength range may be accepted and converted for detection.
- the present solution is different in several ways. Firstly, it is able to cover a much larger spectral range in a single channel, eliminating the need for use of multiple channels.
- US 4,980,566 describes an ultrafast time-resolved spectrometer.
- an IR pico/femtosecond pulse having interacted with a sample is overlapped with another pulse inside a nonlinear crystal to upconvert the IR pulse.
- the pulses and the arrangement are optimised to achieve a short ( ⁇ 1 mm) collinear phase matching.
- the upconverted IR pulse then enters a traditional spectrograph via an entrance slit.
- the invention provides an infrared upconversion spectrometer for determining a mid-IR spectrum of received infrared light, wherein the spectrometer comprises:
- a laser arranged to couple into the nonlinear material in at least substantially the same direction of propagation as, and in spatial overlap with, in-coupled infrared light;
- nonlinear material, the first optical element, and the laser are configured for non-collinear phase-matching in-coupled mid-IR light and upconverted light to be transmitted by the nonlinear material;
- a dispersive element preferably a diffraction grating, arranged to disperse upconverted light such that different wavelength sub-ranges of the
- upconverted light are imaged onto different pixels of the detector
- a spectral resolution of the spectrometer is determined by the laser, the dispersive element and a virtual slit being a spatial overlap between the laser light and the in-coupled infrared light inside the nonlinear material.
- in-coupled radiation refers to the part of the incoming radiation that propagates in the nonlinear material .
- Phase-matching should in the context of this document be understood as either true phase-matching or quasi phase-matching, e.g. as obtainable through poling or orientation patterning .
- the invention uses angular dependent (non-collinear) phase-matching in the upconversion, this is advantageous since it allows for both a broader spectral range to be upconverted (and detected, i.e. more spectral information) and for a larger angular range to be upconverted (using more received light, i.e. better SNR).
- the upconverted light has a concentric pattern with radii as a function of wavelength, i.e. each constituent frequency will emerge with a different angle, i.e. a dispersive effect.
- the wavelengths of the incoupled infrared light that is upconverted preferably lies in a first wavelength range in the mid-IR spectrum.
- the upconverted light lies in a second wavelength range with shorter wavelengths.
- nonlinear materials are suitable for use in the infrared spectrometer. Selection criteria include the nonlinear coefficient at the respective wavelengths, the absorption, and the obtainable phase-matching properties.
- "imag ing the..A..onto .. B” means that A lies in the object plane of some imag ing optics and B lies in the image plane.
- the imaging optics might include cylindrical elements or elements with d ifferent focal length along d ifferent axes, so that A is only imaged along one transverse direction .
- the dispersive element is an element that spreads the light into its different wavelength constituents.
- the d ispersive element is such that it spreads the beam out in its wavelength constituents in the X-Z plane, with the Y-axis being normal to this plane.
- the d ispersive element is a diffraction grating such as a reflection or transmission grating, with its grating lines aligned parallel to the Y-axis.
- the dispersive element is a prism or a holographic element, also oriented so that it spreads the light in the X-Z plane.
- the dispersive element is mostly referred to as a grating, since the orientation of a g rating and the plane into which it spreads a beam is clear and concise by referral to the grating lines.
- the other dispersive elements mentioned above may be substituted with a correspond ing orientation .
- the dispersion element is preferably placed in a infinity corrected plane (in some circumstances also referred to as a Fourier plane) of the imag ing optics.
- the dispersive element may form part of the imag ing optics, e.g . a reflective grating can be shaped like a concave (cylindrical) mirror.
- the radiation incident on a pixel of the spatially resolved detector will correspond to only a part of the second wavelength range of the upconverted rad iation, this is the wavelength sub-range.
- the nonlinear optical process produces upconverted radiation in a ring pattern with varying radii as a function of wavelength.
- this ring pattern has been recorded and analysed by means of a 2-dimensinal detector, typically a CCD camera.
- this ring pattern is dispersed by the dispersive element, such as a grating with lines along the Y-axis).
- the dispersive element such as a grating with lines along the Y-axis.
- the rings becomes displaced in the x-direction with an amount proportional to the wavelength of each ring.
- the dispersed radiation is focused by a spherical lens or mirror, and each ring collapses into a point with a position depending on its wavelength.
- the ring pattern of the upconverted light becomes a line of points and the detector is preferably a line-detector.
- the application of a line-detector will have several beneficial aspects in connection to mid-IR upconversion spectroscopy (here listed in no particular order) :
- Line detectors are commonly available at telecommunications wavelengths: 1.5 Mm whereas 2D cameras at 1.5 Mm is prohibitly expensive for many industrial applications.
- Line arrays allow us to build upconversion devices where the upconverted light ends up at 1.5 Mm rather than in the 0.8-1.0 Mm range. This is relevant for 5-25 Mm spectroscopy because a larger selection of upconversion crystals can then be used; many relevant mid-IR upconversion crystals have a high absorption loss in the 1 Mm range, but significantly better transmission characteristics at longer
- wavelengths e.g. 2 Mm.
- the invention will substantially reduce the post processing time since, in most embodiments, ID rather than 2D signal processing is needed .
- ID rather than 2D signal processing is needed .
- a better S/N is expected due to reduced detector noise (coming from the individual detector elements) and more signal on each detector element.
- the spectral resolution obtainable is, to at least some extent, decided by the phase-match condition of the non-linear material .
- the spectral bandwidth addressable is related also to the phase-match condition .
- the spectral resolution is determined by the effective size of the virtual slit (which relates to the laser beam diameter as described above) and the imaging to the grating (i.e. how much of the grating area is illuminated) and grating periods.
- the spectral resolution is (to some extent) not set by the length of the nonlinear material through the phase-match condition, but by the size of the virtual slit formed by the overlap of the incoupled infrared light and the laser and by the grating design. It further allows for a broader spectral coverage, as a larger part of the upconverted spectrum can be measured.
- the centre of this overlap in the direction of propagation that is positioned in the object plane of the imaging optics is the centre of this overlap in the direction of propagation that is positioned in the object plane of the imaging optics.
- the upconverted light is internally reflected inside the nonlinear material, in which case it is preferably an output end of the nonlinear material that is positioned in the object plane.
- the infrared upconversion spectrometer according to the first aspect is further characterized in that it does not involve an entrance slit to the spectrometer.
- the nonlinear crystal is preferably shaped and positioned such that the spatial overlap between the laser light and the in-coupled infrared light inside the nonlinear material is at least 1mm long, such as at least 2mm, 5mm, 7mm or 10mm long, or preferably at least 15mm or 20mm long in the direction of propagation.
- the laser is an asymmetric diode laser with an asymmetric emitter area, preferably a broad area laser (BAL) or a tapered diode laser, and with an aspect ratio of more than 3, such as more than 5 or 10.
- the fast axis (narrow axis) of the d iode laser is preferably arranged perpendicular to the lines on the diffraction grating .
- BAL or tapered diodes are usually not applicable for upconversion processes due to the typically strict req rindments for mode q uality in both d irections.
- the mode quality requirements in the direction of the grating lines can be sacrificed for the compactness and high power at low costs of such diode lasers.
- the diode laser is preferably focused onto the nonlinear material in a narrow, hig h beam alig ned parallel to the grating lines.
- the spectral resolution of the upconversion spectrometer will be limited by the spectral bandwidth of the mixing laser.
- the asymmetric d iode laser Since the asymmetric d iode laser has good mode q uality in the one direction, it can be coupled into a very narrow crystal . The incoupled infrared light and the diode laser light will then experience total internal reflection in the short direction of the crystal and come out of the narrow end-facet.
- the end- facet of the crystal will be the effective slit of the spectrometer.
- the nonlinear material is a nonlinear crystal with a length (z-axis) to thickness (x-axis) ratio of more than 20.
- This is advantageous since it ensures a long thin crystal in which internal reflections act as a waveguide limiting the size of the virtual slit, which in this case will be the end facet of the crystal .
- the narrow-crystal design can be implemented in an intra-cavity setup as long as the crystal width in the narrow direction is wider than the mixing laser beam. The total internal reflection will restrict the upconverted light to be emitted effectively only from the end facet plane. This will remove the angular spreading of the non- collinear generated upconverted signal and allow for improved spectral resolution even with nonlinear crystals longer than 20 mm.
- a variation of the narrow-crystal design is the wedged crystal design where the width of the crystal is designed e.g . to follow the divergence of the Gaussian mixing laser beam inside the nonlinear crystal.
- the width of the crystal is designed e.g . to follow the divergence of the Gaussian mixing laser beam inside the nonlinear crystal.
- the nonlinear material is poled at a two or more different superposed frequencies. Poling the nonlinear material with several superposed frequencies allows for the wavelengths that would otherwise only obtain phase matching at large angles to be upconverted also a small angles - i.e. reduces the need for the angular dependent phase matching in order to obtain a large spectral range in the upconversion. This has the further advantage that the width of the virtual slit will be smaller since the angles of the upconverted light will be smaller. In the alternative, the same effect may be obtained using two nonlinear crystals cut at different angles and bonded together.
- the invention may be combined with the multichannel techniques described in WO 2015/003721 to form a multichannel infrared upconversion spectrometer by selecting and mounting the nonlinear material appropriately.
- the solution of the present invention has a very large angular acceptance of incoming infrared light.
- Especially large wavelengths, such as 10+ Mm may be upconverted when coming in at angles as large as 55°-60° external to the nonlinear material when using small wavelengths for the mixing laser.
- the k-vectors of the 10+ Mm are short in relation to those of a small wavelength mixing laser, hence the angle of the upconverted light internal in the nonlinear material will not diverge more than smaller IR wavelengths at smaller angles.
- This is advantageous as it increases the bandwidth of the spectrometer as well as the amount of lig ht used in the measurement and thus S/N .
- the imaging optics comprises a cylindrical lens or a concave cylindrical mirror for focusing the dispersed upconverted radiation on the detector, and wherein the detector is a 2-dimensinal detector.
- the ring pattern will no longer collapse to a line. Instead, at the detector, the X-axis will depict wavelength and the Y-axis will depict the angular distribution the upconverted light from the nonlinear material, i .e. the resulting image may be recorded by a 2-d imensional detector.
- the optics for coupling of the mid-infrared l ight to the non-linear crystal is exchanged with astig matic optics such that positions in the y-axis are transferred to ang les in the nonlinear crystal and further to positions in the y-axis on the detector.
- an infrared spectrum is obtained for each position along the y-axis from the input signal .
- the imag ing optics comprises a d iverging lens followed by a converg ing lens for expanding and collimating the upconverted light, respectively; wherein the d ispersive element is positioned to receive the collimated
- the d iverg ing lens and the converging lens allows for a sig nificantly shorter spectrometer.
- the infrared upconversion spectrometer further comprises means for preventing the upconverted light having the largest angles in the x-axis direction from being dispersed by the d ispersive element.
- These means may be an aperture or a skimmer arranged before the dispersive element, or the size and position and/or orientation of the d ispersive element so that the largest angles in the x-direction misses the dispersive element. This is advantageous since it leads to a smaller virtual slit or aperture and thereby a larger spectral resolution.
- the invention provides a method for determining a mid-IR spectrum of received infrared light, comprising:
- nonlinear material the first optical element, and the laser are configured for non-collinear phase-matching in-coupled mid-IR light and upconverted light in a second wavelength range to be transmitted by the nonlinear material;
- a spectral resolution of the spectrometer is determined by the laser, the dispersive element and a virtual slit being a spatial overlap between the laser light and the in-coupled infrared light inside the nonlinear material .
- the gist of the invention is the combination of non-collinear phase- matching and imaging of the resulting ring-pattern via a dispersive element. This allows a simple and efficient set-up which can detect a broader spectrum of incoming IR light that prior art spectrometers. In particular, the invention can be used to detect spectra of incoherent IR light.
- Figures 1A illustrates an embodiment of the invention
- IB is a close-up of the nonlinear material
- Figures 2-6 illustrates different layouts of the spectroscopy part of the
- FIG. 7 and 8 illustrates different layouts of embodiments where the
- Figure 9 and 10 illustrates different embodiments of a multichannel spectrometer according to embodiments of the invention.
- Figures 11A and B are calculated curves showing the phase matched upconverted wavelength as a function of angle of the upconverted light relative to the upconversion laser for angles external to the nonlinear material (11A) and angles internal in the nonlinear material (11B).
- Figures 12A and B are plots of the intensity distribution of upconverted light when propagated (back or forth) to the center of the non-linear crystal illustrated in Figure 12C.
- Figures 13A-C are contour plots showing the phase match curves as they can appear in the plane of the grating.
- the grating considered here is a 25 mm grating placed at an angle of 60° to the Z-axis. In this example the grating acts as a spatial aperture removing larger angles in the x-direction, whereas larger angles in the y-direction
- Figure 1A illustrates part of an embodiment 10 of the infrared upconversion spectrometer according to the invention used in the following for explanatory purposes.
- solid and dashed lines represent different wavelengths ⁇ and ⁇ 2 of received polychromatic infrared light, and are used to show and explain the different radiation paths for different wavelengths.
- the first optical element for coupling the received infrared light 1 into the nonlinear material 3 is here embodied by a spherical lens 2.
- Preferred choices for the nonlinear material are various nonlinear crystal, such as periodically-poled Lithium Niobate (PP: LN), aperiodically-poled Lithium Niobate (AP:LN), AgGaS 2 , AgGaSe 2 , OP:GaAs, OP:GaN, BNA, and DAST.
- the incoupled light is overlapped with a beam 9 (see Figure IB) from a laser 11 coupled in via mirror 12, and with proper orientation or poling of the nonlinear material, a nonlinear optical process such as sum frequency generation (SFG) converts the incoupled light to upconverted light 13 having a shorter wavelength.
- a nonlinear optical process such as sum frequency generation (SFG) converts the incoupled light to upconverted light 13 having a shorter wavelength.
- the angular dependent (non-collinear) phase-matching relied upon in the present invention means that the phase matching condition will be fulfilled for different angles of incidence, ⁇ , for different wavelengths.
- ⁇ for different wavelengths.
- ⁇ 2 ⁇
- the phase matching condition will be fulfilled at smaller angles of incidence.
- Light with wavelength ⁇ but a small angle of incidence will not fulfil the phase matching condition and will thus not be upconverted.
- the spatial distribution of the resulting upconverted light will therefore form a concentric pattern with radii as a function of wavelength, i.e. each constituent frequency will emerge with a different angle, similar to dispersion in a prism or a grating .
- the upconversion process can take place through the entirety of the overlap inside the nonlinear material, here equal to the length L of the nonlinear material . This means that a photon converted near the input can travel along with its new angle as compared to a photon with the same wavelength converted near the exit of the nonlinear material .
- the upconverted light 13 is transmitted by the nonlinear material, and the overlap between incoupled light and laser inside the nonlinear material should be imaged onto the detector 8.
- this imaging is performed by imaging optics comprising lenses 4 and 7.
- the dispersive element in the form of grating 6 with the grating lines in the Y d irection is positioned .
- the dispersion causes the rings corresponding to d ifferent wavelengths to be displaced d ifferently in the X-direction, and the focusing by lens 7 results in a "line of points", each point corresponding to a ring of d ifferent wavelength .
- the referral to rings and points corresponding to a d istinct wavelength is used for the purpose of illustration only.
- the received infrared light will contain a continuous wavelength spectrum and the "ring pattern" and "line of points" will be continuous features as well .
- a line-detector could be e.g .
- the Xlin detector series from Xenics includes multiplexed InGaAs line arrays with 1024 or 2048 pixels and a 12 ⁇ pitch .
- the detectors are eq uipped with one-stage Peltier cooling ; three-stage cooling is also available on an optional basis.
- 1024-pixel detector a line rate of 40 kHz can be achieved ; whereas with the 2048-pixel detector, this value is 10 kHz.
- the infrared upconversion spectrometer of the invention consists overall of an infrared signal input, represented by a set of mixed wavelengths, an upconversion module that converts the infrared signal to higher energy levels, and a
- spectroscopic part that disperse the upconverted signal further and measure the specific amount of each spectral component.
- the input set of wavelengths are upconverted into a set of upconverted wavelengths represented by full, dashed and dotted lines respectively.
- the illustrations track the trajectories of each wavelength.
- 1 and 5 are optional filter elements to separate the signal components from noise and background.
- 2 is a focusing element to match the incoming infrared signal with the upconversion process in the nonlinear material 3.
- the upconverted light is collimated by the focusing element 4 and diffracted by the grating 6.
- the diffracted light is collected by the focusing element 7 and detected by detector 8.
- the layout 30 illustrated in Figure 3 is similar to 20 described above, with a different configuration of the focusing element for the upconverted signal.
- a diverging element, 14, together with a converging element 4 expands and collimate the upconverted signal before it reaches the grating. This results in a more compact construction of the spectrometer
- the layout 40 illustrated in Figure 4 is similar to 30 described above, with a general dispersive element, 6.
- This could be a reflection grating as in 30, or other such as a transmission grating, a prism or a holographic element.
- the layout 50 illustrated in Figure 5 is similar to 20 described above, where the focusing elements for the upconverted signal are included in the dispersive element, 6.
- the layout 60 illustrated in Figure 6 is similar to 50 described above, with a 4f- scaling of the upconverted signal by focusing elements 4 and 7.
- the dispersive element, 6, includes further a design that allow for optimal flat field detection at detector 8. In the following, different layouts for intracavity upconversion are described in relation to Figures 7 and 8.
- Figure 7 illustrates a layout having three high reflective mirrors, 16, 17, and 18, that together create a laser cavity 70.
- a gain medium, 19, is placed such that the gain support lasing inside the cavity.
- the laser crystal 19 is pumped by a diode laser through the dichroic mirror, 18.
- a focusing element 21 scales the field inside the laser cavity the nonlinear crystal 3 converts the incoming infrared signal to a higher photon energy output through a nonlinear sum frequency process between the intracavity laser field and the incoming infrared light.
- the infrared light is coupled in through the dichroic mirror 16 and out through the dichroic mirror 17.
- the layout of the laser cavity 80 illustrated in Figure 8 is similar to 70 described above, but the infrared light is coupled to the crystal with an intracavity parabolic mirror 22, where the laser beam pass through a hole in the parabolic mirror.
- the upconversion output is done through the dichroic mirror 16.
- Figure 9 shows an embodiment of a multi-channel infrared upconversion spectrometer 90 using a technique which may be useful in combination with other embodiments of the invention.
- an angular dependence of the phase- matching conditions within the nonlinear material 3 is used to form different upconversion channels. This is achieved by propagating the in-coupled infrared light along multiple paths within the nonlinear material 3. In this way, different spectral ranges of the infrared light are matched by the different phase-matching conditions along the different paths of the in-coupled light.
- the nonlinear material 3 is rotatably mounted, so that a rotation of the nonlinear material according to arrow 91 directly changes the angle.
- By rotating the nonlinear material 3 between e.g. two rotational positions different up-conversion channels may be formed sequentially in time. The number of channels may easily be increased by using more rotational positions.
- the nonlinear material 3 By rotating the nonlinear material 3 between e.g. two rotational positions, different up-con
- demultiplexer is the rotational mount of the nonlinear material 3, which may selectively couple the infrared light into a first up-conversion channel, a second up-conversion channel, and any other up-conversion channels. Detection of light from the respective up-conversion channels may then be performed by time-gated detection while the nonlinear material 3 dwells in the respective rotational position . Thus, lig ht incident on the detector (not shown in this figure) at any given time will only be the up-converted light corresponding to one channel . The skilled person will realize that a continuum of channels may be achieved in this way by scanning the rotational position of the nonlinear material 3.
- Figure 10 illustrates another embodiment of a multi-channel infrared upconversion spectrometer 100 that may be useful in combination with embodiments of the infrared spectrometer according to the invention .
- the nonlinear material 101 comprises multiple regions (here three are illustrated, in the form of periodically poled regions 102, 103, 104), each having different phase-matching conditions.
- Each region may correspond to a single up-conversion channel and be selectable by transverse translation of the nonlinear material 101 accord ing to arrow 105. In this way, each channel may be formed one at a time.
- the demultiplexer is the translational mount of the nonlinear material 101 in this embodiment.
- the upconverted wavelength will vary according to phasematch relations.
- non-collinear phasematch at varying ang les to the laser will lead to different wavelengths being phasematched at d ifferent angles.
- An example calculation of a periodically poled Lithium Niobate crystal gives the relation shown in Figure 11A expressing the crystal external ang les of the upconverted infrared light.
- Figure 12C illustrates a nonlinear crystal 3 of length L, in which a mixing laser field 9 and a plane mid-infrared sig nal are overlapped .
- the mid-infrared sig nal is a plane wave extending over the full crystal and is not illustrated here.
- the virtual slit 15, is computed from the shape of the mixing laser, the incoming field and the collective shape of the upconverted light 13 at a specific ang le ⁇ as seen from the center part of the crystal in the z-direction . I .e.
- a convolution of a top hat and a Gaussian d istribution that is the width of the virtual slit is a combination of the width of the mixing laser and the angular spread of the upconverted signal in the x-d irection .
- the angle ⁇ of the upconverted lig ht inside the crystal plays a crucial role in determining the virtual slit size in the crystal .
- the infrared light being upconverted is incoherent light coming from e.g . a large thermal source such as a hot filament, which is able to fill the non-linear crystal with infrared light from all ang les in the entire volume of the laser. Since the angles of the infrared light are quite large, it is common to have total internal reflections on the facets in the x and/or y d irections. This helps sig nificantly in filling the whole length of the crystal with infrared light.
- An external angle at around 2 degrees correspond to an internal crystal angle of about 1 deg ree (since the refractive index of PPLN is about 2) .
- An accurate calculation g ives the curves in Figures 11A and B showing the phase matched upconverted wavelength as a function of angle of the upconverted lig ht relative to the upconversion laser for angles external to the nonlinear material ( 11A) and angles internal in the nonlinear material ( 11 B) .
- the calculation is performed for a PPLN crystal with a poling period of 22 ⁇ and a temperature of 60 °C.
- the plot shows the ang le of optimal conversion efficiency, although each wavelength will be upconverted in a range of angles near this maximum .
- the ang les external to the crystal ( 11A) are relevant for designing size and focal length of lenses/mirrors and g ratings.
- the internal crystal ang le ( 11 B) plays an important role in determining the size of the "virtual slit" in the center of the crystal .
- a single wavelength is modelled as a plane wave coming from a single non- collinear angle that may be phase matched (Fig . 12A), or not perfectly phase matched ( 12B) .
- the plots show example calculation for a laser spot radius of 90 ⁇ " ⁇ , and an internal ang le of 1°, a crystal length of 20 mm . The longer the upconverted crystal is, and the larger the internal angle of propagation is, the larger the virtual slit will appear to be. It is important to note that only the field propagated forward to the center of the crystal has a physical presence in the virtual slit, as opposed to the field generated by SFG later in the crystal .
- the intensity for all angles and corresponding phase match cond itions should be added .
- the IR source is incoherent, such as light from a hot filament, it is the intensities that should be added .
- the IR source is a spatially coherent source, the fields should be added before calculating the generated field .
- Figure 12A is calculated as the convolution of a tophat function and a Gaussian function, whereas Fig ure 12B is the same convolution but added with d ifferent phase contribution depend ing on the z-coordinate from which it stems, as outlined in Figure 12C.
- the tophat function orig inates from the fact that the light is generated at d ifferent ang les and z-positions in the crystal and the Gaussian distribution is the product of the incoming plane infrared wave and the mixing laser.
- the total amount of light is an integ ration of all the ind ivid ual intensity distribution over all incident angles with correspondingly varying phasematch condition . This integration should be done separate for each wavelength under consideration .
- the grating with 1200 lines/mm has a wavelength dispersion of around 0.74 nm/mrad .
- This uncertainty in ang le of incident light is a fundamental limiting factor of how the achievable resolution in the spectrometer.
- the laser is 1064 nm, and the IR wavelength is 3500 nm, then up is 816 nm .
- bandwidth in units that are energy-equivalent such as MHz or cm -1 , where the bandwidth is conserved in the wavelength transformation process (fact of energy conservation) .
- an optical component such as a lens.
- the lens should be at a position to catch all the light d iffracted in the desired order by the grating, and focus it to a line on the detector.
- the 25 mm grating is high enough to capture all the lig ht in the vertical direction, but since it is tilted 60 degrees it is not wide enough to capture the light at large angles in the horizontal plane.
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